Fixed Linux thread safety when writing .png files. The Cairo
graphics library (http://cairographics.org) used to draw
.png images on Linux is not thread safe. OEDepict TK now
performs its own mutual exclusion when calling into the Cairo
graphics library to ensure thread safety. It is strongly recommended
to use the .svg file format when speed and server throughput
is a concern. Web browser support for .svg has improved
significantly in recent years and it is more effective at
representing molecule graphs since they are mostly vector
graphics. The only time .png is preferable over .svg
is when rendering heavy raster graphics like property maps from
Grapheme TK.

The reaction role of hydrogens will no longer be thrown away during
the depiction process. Specifically, all information present on
explicit hydrogens will be preserved during the depiction process.

OEDepict TK generated .pdf files opened by several PDF
viewers would show the file as edited due to an internal
representation problem. PDF viewers should no longer modify
OEDepict TK generated .pdf files.

OEDepict TK generated .pdf files should no longer cause
the following warning messages when read into Adobe Acrobat:

The font 'Arial-Bold' contains a bad /BBox.

The Helvetica font will now be used as the default font in
.pdf files. The OEFontFamily.Arial
constant will now map to Helvetica as well in .pdf files.